In the oil and gas industry, “well ranging” or “wellbore surveying” is the art of determining exactly where an underground well is so that the next well can either intersect or avoid it, as needed. Only under ideal conditions will the path of a drilled hole follow the original dip (or inclination) and azimuth (or direction) established at the top of the hole. It is more typical that the borehole will be deflected from the intended direction as a result of layering in the rock, the variation in the hardness of the layers, and the angle of the drill bit relative to these layers.
Because one of the purposes of a borehole is to obtain information in the third dimension—i.e. at depth—the location is just as important as the information itself. Most often the information consists of the geology of the drill core or assays of the core at selected depths. If the hole has deviated significantly, then that information cannot be properly assigned to a location in 3-dimensional space beneath the earth's surface. Conclusions about geological structure or models of the size, shape, tonnage and average grade of ore-bodies based on the ‘mis-placed’ information will be incorrect.
Furthermore, in crowded fields it may be necessary to avoid drilling in already tapped payzones and/or avoid existing wells, or it may be desired to intersect a given well at a particular depth and angle. Indeed with unconventional oil development, unusual well configurations are often needed to realize the value of a play and may be the norm rather than the exception.
Thus, determining the distance and direction between wellbores is critical in many applications, in addition to being used for relief-well applications, parallel twinning for developments such as steam-assisted gravity drainage (SAGD) and coalbed methane (CBM) horizontal-to-vertical intersections. As more fields mature, ranging technology can be used in other applications such as plug and abandonment wells, frac recovery, and for in-fill applications where anti-collision and relative wellbore positioning between multiple wells is of concern.
Although a proven technique, conventional wellbore surveying methodology acquires a discrete number of positional measurements along the well path, which suffer a compounding error from each survey station to the next, generating a positional ellipse of uncertainty (EOU). Because of the cumulative and systematic errors inherent in measurement-while-drilling (MWD) or gyroscopic tools, the measured survey coordinates of the wellbore will have increasing uncertainty with depth, making it practically impossible to accurately steer a well to target by relying solely on survey data of the drilling and target wellbores.
Magnetic directional surveys use measurements of the Earth's field derived from sensors in the survey tool to establish the orientation of the tool with respect to the directional reference defined by the Earth's magnetic field vector. The accuracy of magnetic surveys is compromised as a result of variations and local distortions in the reference magnetic field. Magnetic interference may be defined as corruption of the geomagnetic field by a field from an external source. This can cause serious errors in measuring hole direction (azimuth). Potential sources of magnetic interference are:                Drillstrings        Adjacent wells        Casing shoes        Magnetic formations        “Hot spots” in nonmagnetic drill collars        variations in the magnetic field caused by different rock formations in the earths crust,        variations in magnetic field caused bby solar wind or solar flares.        
Although all the previous error sources may compromise the magnetic survey's quality, drillstring (axial) interference is probably the most common and frequent cause of errors in hole direction. The drillstrings may be regarded as a steel-bar, dipole magnet. The normal approach for magnetic survey tools is to place the survey sensor within sufficient quantity of nonmagnetic drill collars in the bottomhole assembly (BHA). Azimuth measurement errors are minimized by virtue of their distance from the interference source. Magnetic interference diminishes proportionally with the inverse of the square of the distance from the source. However, the bar-dipole-magnet analogy is simplistic. There is evidence that downhole drillstring magnetism may be much more complex, even dynamic in nature. In practice, it may be hard to remove interference completely.
There are several techniques to correct the effects of magnetic interference of crustal and solar anomalies, one of which is “in field referencing” or “IFR”. In-field-referencing is the name given to the practice of measuring the geomagnetic field at, or close to, a drilling site. Measurement of the local geomagnetic conditions during surveys can significantly reduce survey errors and directional uncertainty. IFR techniques thus have the potential to improve the accuracy of magnetic surveys.
The best insurance against crustal anomalies is a site survey to measure the local magnetic parameters in real-time to map the local anomalies, and apply them as corrections to one of the global models. This is known as “IFR1” herein.
The Earth's magnetic field also varies with time due to solar-driven external fields that can be both regular daily or diurnal variations, as well as irregular disturbances in the magnetic fields caused by solar flares. One enhancement of the IFR method is to determine the rapidly varying external field using nearby observatory data. This enhancement is referred to as Interpolation In-Field Referencing (IIFR or “IFR2”). However, the method does require that there be such local observatories and to date not all fields have them.
Applications with IFR1 vary in their work-flow. Sometimes magnetic data at the wellhead is used (FIG. 1A) or a single point at the bottom hole assembly is used (FIG. 1C). Sometimes the reference values at the well head are simply replaced by more accurate IFR1 values taken at various sections of well. FIG. 1B. The most accurate method is to compute a geomagnetic reference value for every downhole survey, as shown in FIG. 1D. Although more accurate, this method is labor intensive, requiring cessation of drilling while the needed magnetic data is collected, and also being compute intensive.
Thus, there remains a continuing need in the art for ever-improved borehole surveying methods, systems and devices to increase the accuracy of borehole surveying. The ideal method would be fast, easy to use, cost effective, but without sacrificing accuracy.